The present invention relates to a mode discrimination circuit for discriminating the broadcasting mode of, for example, TV sound multiplex broadcasting.
In recent TV broadcasting, developments in the TV multiplex broadcasting system are enthusiastically made, which superposes other signals such as data signals on original TV broadcasting signals.
A TV sound multiplex broadcasting system is an example of such TV multiplex broadcasting systems. This TV sound multiplex broadcasting system simultaneously broadcasts the original sound signals with the other sound signals. Thus, this system broadcasts two different types of sound signals, and the sound channels are divided into a main channel and a subchannel for this purpose. The broadcasting mode is differentiated according to the contents of the sound signals to be inserted into the respective channels. There are two broadcasting modes: the dual-sound broadcasting mode and the stereophonic broadcasting mode. In the dual-sound broadcasting mode, sound signals of different contents are supplied to the respective channels. The bilingual broadcasting mode is an example of the dual-sound broadcasting mode. The bilingual broadcasting mode is adopted, for example, for a movie program in a foreign language. In this case, the sound signals of the Japanese language are supplied to the main channel, and the sound signals of the foreign language are supplied to the subchannel. In the stereophonic broadcasting mode, sum signals (L+R) of left signals (L) and right signals (R) are supplied to the main channel. On the other hand, difference signals (L-R) between the two signals are supplied to the subchannel.
Transmission and processing of the signals in a TV sound multiplex broadcasting system will briefly be described first. TV sound multiplex signals include pilot signals as reference for discrimination of the broadcasting modes at the receiver side in addition to the sound signals of the main channel (to be referred to as main channel signals hereinafter) and the sound signals of the subchannel (to be referred to as subchannel signals hereinafter) as has been described above. Carriers of the main channel signals are the same as those for the original sound signals. These carriers will be referred to as main carriers hereinafter. The main carrier frequency is higher than the frequency of the video signal carriers by 4.5 MHz. Carriers of the subchannel signals are those having a frequency higher than the main carrier frequency by about 31.5 MHz. These carriers will be referred to as the subcarriers. As the pilot signals, sine waves having a frequency of, for example, 922.5 Hz are used in the dual-sound broadcasting mode, and sine waves having a frequency of, for example, 982.5 Hz in the stereophonic broadcasting mode. As carriers for these pilot signals are used signals having a frequency higher than the main carrier frequency by about 55.1 kHz. These carriers will be referred to as pilot carriers.
The subcarriers are frequency-modulated by the subchannel signals. The pilot carriers are amplitudemodulated by the pilot signals. The frequency-modulated subchannel signals (to be referred to as modulated subchannel signals hereinafter) and the amplitudemodulated pilot signals (to be referred to as modulated pilot signals) are mixed with the main channel signals to provide composite signals. The main carriers are frequency-modulated by the composite signals, and the frequency-modulated main carriers are transmitted as the TV sound multiplex signals.
At the reception side, demodulation of the received TV sound multiplex signals is performed to provide the composite signals described above. These composite signals are directly used as the main channel signals and are also supplied to a band-pass filter for separating the modulated subchannel signals and a band-pass filter for separating the modulated pilot signals. The signals separated by the respective band-pass filters are subjected to predetermined demodulation and are obtained as the subchannel signals and the pilot signals. The demodulated pilot signals are supplied to a mode discrimination circuit which discriminates the broadcasting mode. If the frequency of the pilot signals is 922.5 kHz, the mode discrimination circuit discriminates the broadcasting mode to be the dual-sound broadcasting mode. On the other hand, if the frequency of the pilot signals is 982.5 Hz, the mode discrimination circuit discriminates the broadcasting mode to be the stereophonic broadcasting mode. If the discrimination result of the mode discrimination circuit is the dual-sound broadcasting mode, the demodulated main and subchannel signals are supplied to the corresponding speakers for reproduction. If the discrimination result of the mode discrimination circuit is the stereophonic mode, the main and subchannel signals are matricized, are separated into left signals (L) and right signals (R), and are supplied to the corresponding speakers.
Current TV broadcasting includes both TV sound multiplex broadcasting and the conventional monaural broadcasting. In the description to follow, the term TV sound multiplex broadcasting will be used in a broad sense to include monaural broadcasting as well. Dual-sound broadcasting and stereophonic broadcasting will be termed multiplex broadcasting as opposed to monaural broadcasting.
In monaural broadcasting, only the main channel signals of multiplex broadcasting are transmitted, and the subchannel signals and the pilot signals are not transmitted. Therefore, the discrimination of monaural broadcasting can be performed through the detection of the presence or absence of the pilot signals. Considering the points described above, the mode in which no pilot signals are transmitted may be considered as a mode in which a special type of pilot signal which has zero frequency is present.
The configuration of a conventional mode discrimination circuit for discrimination of the broadcasting mode through detection of the pilot signals will now be described. The conventional mode discrimination circuit typically has a first discriminating means for discriminating whether the broadcasting mode is the multiplex broadcasting mode or the monaural broadcasting mode, and a second discriminating means for discriminating, if the broadcasting mode is the multiplex broadcasting mode, whether the multiplex broadcasting mode is the dual-sound broadcasting mode or the stereophonic broadcasting mode. These two discriminating means basically are constituted by using phase locked loops (to be referred to as PLL hereinafter). The pilot signal is supplied to one input terminal of a phase comparator (to be referred to as a first phase comparator hereinafter) while the oscillation signal from a voltage controlled oscillator (to be referred to as VCO hereinafter) is supplied to the other input terminal of the first phase comparator. Then, at the output terminal of the first phase comparator is obtained a dc voltage, which is proportional to the frequency and phase differences of the pilot signal and the oscillation signal, in accordance with the so-called S-shaped characteristics. This dc voltage is supplied to the VCO as a control voltage. Then, at the output terminal of the VCO is obtained the oscillation signal which is frequency- and phase-synchronized with the pilot signal. It is to be noted, however, that the VCO freely oscillates since the frequency of the pilot signal is zero in the monaural mode.
The first discriminating means has a phase shifter for shifting the phase of the oscillation signal from the VCO through 90.degree., and a second phase comparator for comparing the phase of the output signal from the phase shifter with that of the pilot signal. With the mode discrimination circuit of this configuration, in the monaural broadcasting mode, since the frequency of the pilot signal supplied to one input terminal of the first phase comparator is zero, the dc voltage output from the second phase comparator becomes low level. In the multiplex broadcasting mode, since the pilot signal supplied to one input terminal of the first phase comparator and the oscillation signal from the VCO are frequency- and phase-synchronized, the dc voltage output from the second phase comparator becomes high level. In this manner, the dc voltage output from the second phase comparator takes different levels depending on whether the broadcasting mode is the monaural broadcasting mode or the multiplex broadcasting mode. The first discriminating means performs the discrimination of the broadcasting mode utilizing this level difference.
The second discriminating means discriminates the dc voltage output from the first comparator with reference to a predetermined threshold voltage so as to discriminate if the broadcasting mode is the dual-sound broadcasting mode or the stereophonic broadcasting mode. Let f0 denote the free-oscillation frequency of the VCO, and f1 and f2 denote the frequency (922.5 Hz) of the pilot signal in the dual-sound broadcasting mode and the frequency (982.5 Hz) of the pilot signal in the stereophonic mode. Then, if the dc voltages, output from the first phase comparator when the oscillation frequencies of the VCO are respectively f1 and f2, are denoted by E1 and E2, the threshold voltage as described above is set to be halfway between the dc voltages E1 and E2. The threshold voltage may be a dc voltage E0 of the first phase comparator when the VCO freely oscillates.
Thus, the conventional mode discrimination circuit has two discriminating means. The final discrimination result is obtained by subjecting the discrimination results from these two mode discriminating means to logic operation, for example.
However, the conventional mode discrimination circuit as described above has the following drawbacks. First, in the conventional mode discrimination circuit, there are differences in the discrimination characteristics with respect to the intensity of the electric field of the broadcasting electromagnetic waves between the first and second discriminating means. More specifically, the lower limit of the intensity of the electric field of the first discriminating means for allowing the discrimination of the broadcasting mode is higher than that of the second discriminating means. In this case, when the first discriminating means can no longer perform the mode discrimination below this lower limit, the state of the output from the first discriminating means becomes the same as that obtainable when the monaural broadcasting mode is discriminated. Therefore, when the intensity of the electric field ranges from the lower limit of the intensity of electric field which allows the second discriminating means to function to the lower limit of the intensity of electric field which allows the first discriminating means to function, the broadcasting mode may be discriminated erroneously as monaural broadcasting mode in the final logic operation, despite the second mode discriminating means correctly discriminates the dual-sound broadcasting mode or the stereophonic broadcasting mode.
Furthermore, the conventional mode discrimination circuit requires a plurality of circuits for constituting the first and second discriminating means which are connected to the PLL in addition to the PLL itself. This makes the overall circuitry complex in configuration and makes the integration of the circuitry difficult.